According to the encryption apparatus and the decryption apparatus disclosed in the Patent Document 1, data is ciphered by a mask “a” and a mask “b”, and by removing the influence of the masks after operations of encrypting or decrypting, data without the masks can be obtained. Thus, by using the mask “a” and the mask “b”, it is possible to prevent the internal data from being estimated and to deter it from being decrypted based on a power analysis.    Patent Document 1: Japanese Unexamined Patent Publication No. 2000-66585 (pp. 7-13, FIG. 4)